1. Field of the Invention
The present invention relates to an NMR probe for use in an NMR (nuclear magnetic resonance) spectrometer and, more particularly, to an NMR probe that produces a magnetic field at the resonant frequency, the strength of the field being maximized in the center of the sample coil.
2. Description of the Related Art
An NMR probe of this kind is disclosed in patent reference 1. FIG. 25 is a schematic diagram of the NMR probe shown in patent reference 1. The NMR probe, 100, has a resonant circuit including a sample coil 101 and five impedance elements 102, 103, 104, 105, and 106. Of these 5 elements, the impedance elements 102 and 103 are connected in series and connected with one end of the sample coil 101. Similarly, the impedance elements 104 and 105 which are connected in series are connected with the other end of the sample coil 101. The impedance element 106 is connected in parallel with the sample coil 101. More specifically, the impedance elements 102, 103, 104, and 105 are transmission lines. The impedance element 106 is a capacitor or inductor.
In FIG. 25, the impedance elements 102-106 have variable impedances. The resonant circuit produces multiple resonance at a first radio frequency f1 and a second radio frequency f2, where f2<f1. Where the impedance elements 102, 103, 104, and 105 are transmission lines, their lengths are adjusted. Where the resonant circuit resonates at the first radio frequency, the adjustment minimizes the strength of the electric field in the center of the sample coil 101.
According to the above-cited patent reference 1, when the resonant circuit resonates at the two frequencies f1 and f2 simultaneously, the minimal point of the electric field strength at the resonant frequency f1 is produced in the center of the coil 101.
[Patent reference 1] U.S. Pat. No. 7,187,176
The subject to be substantially measured in NMR is the magnetic field emitted from the sample to be investigated, the sample being excited by an RF magnetic field. The detected strength of the magnetic field depends on the field strength produced when the sample is excited. Therefore, if this fact is taken into consideration, it is desired that an intense magnetic field be produced efficiently for the RF waves applied to the sample coil.
However, producing the minimal point of the electric field strength in the center of the sample coil as described in the above-cited patent reference 1 is not equivalent to producing an NMR signal that is strong enough to be detected. The NMR signal is generated by the RF magnetic field produced in the position of the sample within the coil.
This is further described by referring to FIGS. 26 and 27. FIG. 26 shows one example of a resonant circuit for detecting an NMR signal, the resonant circuit being designed to resonate at a first RF frequency f1 and a second RF frequency f2 (where f2<f1), i.e., producing multiple resonance.
As shown in FIG. 26, a resonant circuit, 100, has a sample coil 201, two transmission lines 202, 203, three impedance circuits 208, 209, 210, a matching circuit 211 and a connector 212 for the first RF waves, and a matching circuit 213 and a connector 214 for the second RF waves.
Respective one ends of the two transmission lines 202 and 203 are connected with the opposite ends of the sample coil 201. The other ends of the two lines 202 and 203 are connected with the impedance circuits 208 and 209, respectively. The impedance circuit 210 connects the junction 220 between the line 202 and the impedance circuit 208 with the junction 221 between the line 203 and the impedance circuit 209. The connector 212 for the first RF waves is connected with the junction 220 via the matching circuit 211 for the first RF waves. The connector 214 for the second RF waves is connected with the junction 221 via the matching circuit 213 for the first RF waves. The three impedance circuits 208, 209, and 210 maintain the impedances at the two junctions 220 and 221 appropriately.
FIG. 27 shows the results of analysis of the distribution of the electric field and the distribution of the magnetic field produced within the sample coil 201. It is assumed that the shown sample coil 201 is a solenoid coil. FIG. 27 shows a cross section taken axially of the coil. Dotted lines extending from the opposite ends of the sample coil 201 indicate extension lines 215 and 216.
In FIG. 27, the sinusoidal curve 217 indicated by the solid line shows the distribution of the magnetic field strength when the coil is resonating at the frequency f1 (600 MHz). The sinusoidal curve 218 indicated by the dotted line indicates the distribution of the electric field strength when the coil is resonating at the same frequency. As can be seen from these curves, the minimal point E of the electric field strength is not coincident with the maximal point B of the magnetic field strength within the coil 201.
In a solid-state NMR probe circuit, it is advantageous that the sample coil and the axis of the sample are tilted by an angle, known as a magic angle, relative to the axis of the transmission line, and the resulting NMR signal is derived. Therefore, the right and left parts of the sample coil are normally different in mechanical structure. This difference in mechanical structure affects the distributions of the electric and magnetic fields within the sample coil. Consequently, the minimal point of the electric field strength and the maximal point of the magnetic field strength within the sample coil are produced at different locations.
As can be seen from the description provided so far, if the circuit is so designed that the minimal point of the electric field strength is placed at the center of the sample coil, there arises the problem that it is impossible to obtain a uniform and strong magnetic field which is necessary to appropriately extract the NMR signal at the position of the sample.